1. Field of the Invention
This invention relates to a water vapor cryopump cooled by a Gas Balanced Brayton cycle refrigerator, typically having input power in the range of 5 to 20 kW.
2. Background Information
Three recent patent applications assigned to SHI Cryogenics describe gas balanced Brayton cycle expansion engines and a control system that minimizes cool down time from room temperature to cryogenic temperatures. A system that operates on the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a discharge pressure to a counterflow heat exchanger, which admits gas to an expansion space through a cold inlet valve, expands the gas adiabatically, exhausts the expanded gas (which is colder) through in outlet valve, circulates the cold gas through a load being cooled, then returns the gas through the counterflow heat exchanger to the compressor.
Patent application Ser. No. 61/313,868 dated Mar. 15, 2010 by R. C. Longsworth describes a reciprocating expansion engine operating on a Brayton cycle in which the piston has a drive stem at the warm end that is driven by a mechanical drive, or gas pressure that alternates between high and low pressures, and the pressure at the warm end of the piston in the area around the drive stem is essentially the same as the pressure at the cold end of the piston while the piston is moving. Patent application Ser. No. 61/391,207 dated Oct. 8, 2010 by R. C. Longsworth describes the control of a reciprocating expansion engine operating on a Brayton cycle, as described in the previous application, which enables it to minimize the time to cool a mass to cryogenic temperatures. U.S. patent application Ser. No. 13/106,218 dated May 12, 2011 by S. Dunn, et al., describes alternate means of actuating the expander piston. The engines described in patent application 61/313,868 and Ser. No. 13/106,218 are referred to in this application as “Gas Balanced Brayton cycle engines”. This engine has a lot of advantageous characteristics when it is used to cool a cryopanel that is condensing water vapor at temperatures in the range of 110 K to 170 K. The compressor system that is used in this application to illustrate the innovations is described in published patent application US 2007/0253854 titled “Compressor With Oil Bypass” by S. Dunn filed on Apr. 28, 2006.
Starting in the late 1950's a lot of work was done in cryopumping technology to support the space program. U.S. Pat. No. 3,010,220 dated Nov. 28, 1961 by Schueller describes a space chamber with cryopanels cooled by liquid cryogens. U.S. Pat. No. 3,175,373 dated Mar. 30, 1965 by Holkeboer, et al., describes a large vacuum system that has conventional mechanical and diffusion pumps, and liquid cryogen cooled cryopanels. A paper by C. B. Hood, et al., titled “Helium Refrigerators for Operation in the 10-30 K Range” in Advances in Cryogenic Engineering, Vol. 9, Plenum Press, New York (1964), pp 496-506, describes a large Brayton cycle refrigerator having a reciprocating expansion engine capable of producing more than 1.0 kW of refrigeration at 20 K. This refrigerator was developed to cryopump air in a large space chamber. An early small cryopump cooled by liquid nitrogen and a GM refrigerator is described in U.S. Pat. No. 3,338,063, dated Aug. 29, 1967, by Hogan, et al. GM type refrigerators that draw less than 10 kW of input power have dominated the market for cooling cryopanels that pump all gases since then, U.S. Pat. No. 4,150,549 dated April 1979 by Longsworth, is an example. Starting in the early 1970's cryopumping water vapor at temperatures in the range of 120 K to 170 K and capacities of 500 to 3,000 W have been dominated by refrigerators that use mixed gases as described in U.S. Pat. No. 3,768,273 dated Oct. 30, 1973 by Missimer. A more recent patent, U.S. Pat. No. 6,574,978 dated Jun. 10, 2003 by Flynn, et al., describes means of controlling the rate of cooling and heating a refrigerator of this type.
The present application is a departure from present practice of using mixed gas refrigerant refrigerators having capacities of about 500 to 3,000 W at about 150 K to pump water vapor, by using a Gas Balanced Brayton cycle refrigerator which typically circulates helium.